The invention relates to treatment of fluids to remove acid gases, in which the fluid is contacted with a solvent which preferentially absorbs the acid gases. More particularly, the invention is concerned with the regeneration of a solvent of the type specified for reuse.
The term "absorbing solvent", as used herein, shall mean either an aqueous solution of a chemical solvent or solution comprising a combined chemical solvent and physical solvent with some water, which solutions are capable of absorbing acid gases. The chemical solvent part comprises one or more basic amines and may comprise a primary, secondary and/or tertiary amine. Alkanolamines are suitable, especially those having 1 to 4 and preferably 2 to 3 carbon atoms per alkanol radical, while dialkanolamines are particularly advantageous. Typical examples are monoethanolamine, diethanolamine, diisopropanolamine, diethylethanolamine, methyldiethanolamine and mixtures thereof. Other amines which can be used are alkylamines, phenyl alkylamines, alkoxyalkyl and alkoxyaryl amines. Typical of these are methylethyl amine, phenyl ethylamine, and methoxy diethyldiamine. Aliphatic amines having between 2 and 8 amine groups may also be used. Examples of such amines are triethylenetetramine, tetraethylenepentamine, and derivatives thereof.
The physical solvent part comprises at least one component selected from cyclotetramethylene sulphones, aliphatic acid amides, perhalo alkanes, N-alkylated pyrrolidones or N-alkylated piperidones or their derivatives.
The derivatives from the basic sulphone are cyclotetramethylene sulphone or thiophene tetrahydro-1,1-dioxide, which is also known as sulfolane, and should have not more than four, more preferably not more than two alkyl substituents in the tetramethylene sulphone ring. Sulfolane is the preferred species of this class of compounds. Suitable amides are the dialkyl-N-substituted aliphatic acid amides, a preferred species being dimethyl formamide. The alkyl groups directly attached to the nitrogen atom should have from 1 to 4 carbon atoms each, while an acid with 1 to 4 carbon atoms per molecule is preferred. Apart from dimethyl formamide, other species in this sub-class include methyl ethyl formamide, diethyl formamide, propyl methyl formamide, dibutyl formamide, dimethyl acetamide, methyl ethyl acetamide, formamide and acetamide. Suitable perhaloalkanes include perfluoro alkanes such as perfluoro ethane and perfluoro butane as well as perchloralkanes such as perchloro propane. Where an N-alkylated pyrrolidone or an N-alkylated piperidone is used, although the alkyl substitutent on the nitrogen atom may be any alkyl group, alkyl groups with 1 to 4 carbon atoms are preferred, and N-methyl pyrrolidone is particularly suitable.
For the purposes of this invention, carbonyl sulphide (COS) will be considered to be an acid gas.
In a typical process for removing acid gases from a fluid, the fluid is contacted with an absorbing solvent which preferentially absorbs the acid gases, the solvent which has thus absorbed acid gases being referred to as "fat solvent". The fat solvent is regenerated by stripping the absorbed acid gases from it using steam. The stripped, or lean solvent is then recirculated for reuse and the released acid gases are removed for further treatment. In such a process, the major part of the operating costs results from the steam consumption required for the regeneration step.
It is an object of the present invention to improve the efficiency of the regeneration of a solvent of the type specified.